Operando adsorption of heavy metals and their recycling into electrodes for energy storage

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Marcelo AMARO DE ANDRADE,

Institut des Matériaux Jean Rouxel de Nantes, France

Treating wastewater contaminated with heavy metals often relies on passive adsorption using high-surface area materials, but once these adsorbents are saturated, they become hazardous waste themselves. In this work, Hg²⁺ cations uptake by reduced graphene oxide (rGO) was tracked under continuous-flow conditions, using operando techniques to get a realistic picture of the adsorption process. A microfluidic platform was combined with Hg L₃-edge synchrotron X-ray absorption spectroscopy (XAS) to monitor, in real time, how mercury coordinates and loads onto rGO during adsorption. At the same time, an electrochemical quartz-crystal microbalance (EQCM) was used to independently follow changes in mass and viscoelastic properties of the rGO layer under model wastewater flow. This approach allowed to distinguish between strongly bound (chemisorbed) and more weakly coordinated (physisorbed) mercury species, and to observe how these forms changed dynamically as adsorption progressed. ¹

On top of that, the metals captured in these materials can also be used as redox-active components for further applications. Our group previously showed how rGO foams used to capture Hg²⁺ cations from model wastewater (rGO/Hg_ads) can be directly recycled into self-standing electrodes without extra chemical or thermal steps. In H₂SO₄, these electrodes combine double-layer capacitance with redox-based faradaic reactions, resulting in about 33% higher gravimetric capacity compared to pristine rGO (Figure 1 – left). ²

To understand the charge-storage mechanism, operando synchrotron Hg L₃-edge XAS was used, in addition to operando EQCM. Time-resolved XANES revealed two main mercury states: oxidized Hg(II) and a reduced state most consistent with Hg(I). Their concentration profiles show reversible cycling between Hg(II) and Hg(I), which corresponds to the electrochemical features observed in cyclic voltammetry. Hg(II) is consumed at the reduction peak around 0.60 V, and regenerated at the oxidation peak near 0.64 V, starting as early as 0.45 V (Figure 1 – right). This suggests that redox changes are not limited by simple thermodynamics, and the capacity gain comes indeed from the adsorbed cations. L₃-edge jump analysis shows that the total mercury content changes with potential, showing considerable mercury movement in and around the electrode during cycling, which was correlated to EQCM-D measures. Overall, this ReMade project allowed to understand both the heavy-metal capture mechanism, and their later contribution to the electrochemistry by redox changes, helping to advance new ways of combining environmental remediation with energy storage materials.

References:

1. Andrade, M. A., Bugaev, A. L., Skorynina, A. & Douard, C. Tracking Hg²⁺ adsorption by reduced graphene oxide in continuous flow by in situ techniques. J. Environ. Chem. Eng. 13, 118680 (2025).

2. Andrade, M., Crosnier, O., Johansson, P. & Brousse, T. Energy from Garbage: Recycling Heavy Metal‐Containing Wastewater Adsorbents for Energy Storage. Adv. Energy Sustain. Res. 6, (2024).